The desire to reduce emissions of greenhouse gases to the atmosphere led to the development of carbon capture and storage (CCS) technologies that target large, point source emitters of carbon dioxide (CO2). While CCS promises to have a big impact in reducing carbon emissions, even when fully deployed, CCS generally only captures about 50% of current CO2 emissions. The remainder originates from small, dispersed, and mobile sources. In turn, the majority (65%) of these are associated with the transportation sector, which is also experiencing the largest growth in emissions. Emissions could also rise if the amount of oil from unconventional sources, such as tar sands, increases dramatically. Proposed solutions to effectively de-carbonize the sector include electricity and hydrogen (H2) as likely candidates.
De-carbonizing transportation fuels reduce anthropogenic CO2 emissions only if the source of the fuel and the overall production process is carbon neutral. For hydrogen and electricity, the key development is either the full deployment of conventional CCS or a paradigm shift to an electricity grid completely dominated by renewable power systems such as wind, solar, and biomass. Even with these developments, both CCS and renewable power have associated fugitive emissions. The CO2 profile of CCS (40-152 g CO2/kWh) is on a similar scale to renewable systems (12-63 g CO2/kWh). Non-hydro renewable power accounts for 18% of current electricity generation, including nuclear.
The introduction of the necessary changes to the energy infrastructure, either CCS or renewable power, will take time. For example, the de-carbonized solutions also depend on the separate development of infrastructure to transport and deliver the hydrogen or electricity. As a result, “bridge” fuels, e.g., synthetic fuels having a lesser carbon footprint than current fuels, may be necessary to reduce emissions until the energy infrastructure is changed.
Much effort is being spent on developing a sustainable transportation fuel. The exact definition of sustainable generally assumes no fossil CO2 emissions to the atmosphere and reduced emission of category pollutants such as sulfur compounds. An often sited fuel is hydrogen (H2), which produces only water vapor upon combustion. An equally important consideration is how the hydrogen is produced. If it is produced using fossil fuels, currently by stripping natural gas, the fuel will have a fossil CO2 burden. The exact burden will depend on the type of fuel, coal or natural gas, and the management plan, geological sequestration or other. It is unclear how much extra energy will be required to for transportation, handling, and on board use of hydrogen. Ethanol from biomass is also a renewable fuel option but it is unclear at this time how significant are the reductions in fossil fuels.